Drying or curing of liquid coatings on a substrate or web is typically achieved by passing the substrate through a drying chamber, in most cases, a conventional oven (either a floatation or roller supported oven), where the liquid is evaporated and the coating is dried or cured. The oven is heated with heating elements. The heat is passed onto the coating through convection or forced gas flow, typically air. Multiple zones of the oven may be employed to allow flexibility in the temperature adjustment. When organic solvent is used in the coating where explosion and fire can be a potential hazard, regulations impose a maximum concentration of the solvent allowable in the drying chamber to ensure safety of operation. This maximum concentration is defined in terms of a fraction or most often as a percentage (% LEL) of the lower explosive limit (LEL) of the solvent or mixture of solvents removed from the coating undergoing drying or curing. LEL is the lowest concentration where a conflagration or explosion can be propagated from an initially ignited point; LEL is a property of the solvent or solvent mixture, whereas % LEL is just a measure of concentration of a particular solvent or solvent mixture referred to the LEL of that solvent or solvent mixture. The maximum allowed solvent concentration in a given dryer (in terms of % LEL) that a dryer is allowed to safely handle by regulation ultimately does limit substrate or web speed.
The solvent removed from the coating is either condensed into liquid form through a condensation system, or more commonly burned by a thermal oxidation unit (TOX). When a condensation system is used, it often includes a tandem of condensers, typically and needs to operate at a low temperature (below 0° C.) to condensate most of the solvent, with a correspondingly high demand on energy. When a TOX unit is used, there is a maximum amount of solvent per unit time that can go through a TOX due to safety (explosivity, which constrains solvent concentration, and energy release, i.e. maximum operating temperature, which constrains the throughput). This imposes a limit on the solvent content out of the combined streams sent to the TOXs, and therefore ultimately imposes a limit on the maximum substrate speed through the oven.
To make a conventional oven inert, i.e. to use inert gas, rather than the oxygen rich air, could alleviate the potential of explosion and fire, and therefore, increase the solvent throughput a dryer can handle. However, the volumes and flow rates of the inert gas involved in that operation can make it expensive and affect adversely the economic viability of the process.
Another typical issue associated with conventional oven is blistering, i.e., appearance of bubbles in the dried coating. It is caused by rapid bubble growth from gases dissolved or entrained in the coating liquid and from the volatile solvents in the coating, which exhibit a high vapor pressure. To alleviate blistering or to allow the healing of blistering, usually both temperature and speed of the gas (commonly air) is reduced, typically in the first zone(s) of the oven when multiple zone are used.
Another approach to increase the throughput of a conventional oven is to add additional heating zones to that oven. However, the size and volume of conventional ovens makes it difficult to add new zones to retrofit existing installations. Accidents involving explosion or conflagration of solvent laden air above LEL concentration would also involve a greater volume of explosive mixture; Inertization becomes more expensive with the additional zones to inert and may take considerable time (15 to 30 or more minutes). This creates further delays in case of web rupture, which need dryer opening, cleaning, rethreading of the web, and re-inertization.
Another type of dryer that can be used for drying liquid coatings is a plate dryer. They may include heated plates at one or both sides of a moving web. They have been used in pultrusion and other curing processes. In these applications process velocities are low (<30 m/min). Typical flows of the inerted gas-solvent mix are in the range of 1 m/s with low external mass transfer and heat transfer, which makes it not suitable for higher speed coating lines.
U.S. Pat. No. 4,894,927, assigned to Fuji Photo, teaches the benefits of a low volume inerted plate dryer and how the system can include solvent recovery by condensation and how heat can be recovered by placing a heat exchanger between the dryer and the condenser. Furthermore, U.S. Pat. No. 4,926,567, also assigned to Fuji Photo, teaches how the incoming inert stream can be heated by heat exchange with the exhaust of an incinerator where the recovered solvent is burned. Neither patent teaches how the systems are sealed to avoid contamination of the ambient air to the heater and vice versa. Nor do they teach what conditions are needed for the system to be beneficial. Furthermore, both patents consider that the entire dryer exhaust stream undergoes condensation.
One type of plate dryers is designed with internal condensing surfaces, which are sometimes referred to as “gap dryers”. In this type of dryer heat is provided by a hot plate or any other suitable source. The carrier web moves over the plate or close by. Condensation occurs inside the dryer, over a cold surface that creates a concentration gradient that drives significant diffusion of the solvent. U.S. Pat. No. 5,581,905 (and sequels) assigned to 3M teaches substantially horizontal configurations of the plates where the cold surface is kept as close as ˜0.5 cm above the drying wet coating. Condensation occurs on the lower surface of the cold top plate which is grooved such that capillarity drives the liquid out towards the edges where it is drained. No significant convective gas flow occurs inside the dryer apart from that induced by web drag. There is the possibility of solvent dripping over the drying coating, as well as water condensation if air enters the system.
Therefore, there is a need for a dryer that can dry solvent based coating with high efficiency, high throughput and more economical than existing dryers.